1. Field of the Invention
The present invention relates to a communications device, and more particularly to a communications device for communication over wireless channels.
2. Description of the Related Art
Recent years have seen an explosive growth in the number of users of mobile communications facilities such as cellular phone networks. Besides providing voice call functionality, today's mobile handsets are offering more and more capabilities for use as a multifunctional portable device, particularly with an enhanced support of Internet access. This trend is expected to lead to the development of more advanced multimedia services in the field of mobile applications.
In mobile communications, the propagation path characteristics change from moment to moment. As a mobile station moves, its line-of-sight path to the base station is often obstructed by buildings or other terrestrial objects. Such a dynamic nature of path characteristics causes degradation of quality of communication service, and how to overcome this challenge is one of the important issues in the mobile communications technology.
FIG. 20 shows a simplified model of propagation between a base station and a mobile station. Carrier waves transmitted from an antenna of the base station BS travel over two or more different routes before arriving at a mobile station MS. This is known as “multipath,” the phenomenon that results in radio signals' reaching the receiving end via a plurality of paths because of the reflection from objects such as mountains and buildings. When the mobile station MS is moving, carrier waves coming from different directions suffer different amounts of Doppler shift, depending on the angle of arrival. The signal frequency observed at the moving mobile station MS is not constant, but varies since it is a sum of carrier frequency and Doppler frequency. The mobile station MS receives a plurality of such incoming radio wave signals with a broadened spectrum, experiencing significant fluctuations of received signal level, the phenomenon called “multipath fading.”
The fading fluctuations described above result in an increased data error rate in wireless communications. A good estimate of Doppler frequency would allow the system to choose a suitable radio communication technique adaptively and achieve better performance in a given propagation environment. Thus, it would be beneficial if we could estimate the Doppler frequency accurately.
Researchers have investigated techniques for Doppler frequency estimation, and one such example is disclosed in the Japanese undexamined Patent Application No. 10-65610 (1998) (see paragraphs 0007 to 0017, FIG. 1). According to this publication, the average power of each path's despread signal is calculated, and of all those despread signals, the one with the largest power is subjected to the Fourier transform. The resultant power spectrum shows a point with the greatest amount of power, and the frequency of that point is used as an estimate of maximum Doppler frequency.
The above existing technique, however, allows noise components to affect the result of the estimation. Also, an increase in the number of multipath components and the consequent complex behavior of Doppler effects make it difficult to obtain an accurate Doppler frequency estimate.
Among other proposed techniques for Doppler frequency estimation, one method measures the interval of Doppler fluctuations. Another method repeats channel estimation at constant intervals and uses the phase differences between the obtained channel estimates for estimation. With those generally known methods, however, it is difficult to estimate Doppler frequency in a stable and accurate way when the receiver is in such conditions where the signal-to-noise (S/N) ratio is low.